This invention relates to a biaxially stretched, tubular multilayer film which is constituted by two or more layers and in which both surface layers are of a polypropylene resin. More particularly, the present invention relates to a biaxially stretched, tubular multilayer film having both good transparency and good lubricating properties produced by the inflation biaxially stretching method using spherical antiblocking agent particles. The multilayer may be, for example, a 2-layer biaxially stretched film wherein the two layers comprise a polypropylene resin, a 3-layer biaxially stretched film which comprises a core polyethylene resin layer sandwitched between two polypropylene resin layers, and a 5-layer biaxially stretched film which is constituted by two surface layers composed of polypropylene resin, a core layer composed of a saponified product of ethylene/vinyl acetate copolymer, and two interlayers composed of an adhesive resin such as a propylene-butene-1 copolymer respectively provided between the surface layers and the core layer. The present invention is also directed to a method for manufacturing a tubular multilayer film having the above-described constitution.
Stretched films comprising a polypropylene resin are in wide use for packaging. Particularly, multilayer polypropylene stretched films wherein the two surface layers are formed of a polypropylene resin are important because of their excellent properties.
Such stretched films are required to be excellent in lubricating properties in view of adaptability to packaging machines and of lubrication be packaged articles as well as in transparency. This requirement has been satisfied by adding an antiblocking agent together with a lubricant to the whole film, to a single layer film or to a surface layer of a multilayer film. As such antiblocking agents, inorganic silica materials of amorphous and porous form have been used. However, these antiblocking agents seriously decrease transparency when added in an amount sufficient to obtain sufficient lubricating properties, and thus the resulting films are not suitable for packaging use.
Recently, spherical antiblocking agent particles composed of a silicone resin or a non-porous inorganic material have come into use. For example, Japanese Patent Publication Nos. 62-215646, 62-232448, 62-233248 and 1-135840 describe the use of silicone resin fine particles. These antiblocking agents are known to provide good lubricating properties without seriously spoiling the transparency of the films. In case of obtaining a biaxially stretched film by an inflation biaxial stretching method using these antiblocking agents, however, any fold of the unstretched raw sheet formed upon manufacture thereof by a pair of pinch rolls, remains as a white line, unfavorable to use as packaging film. Particularly, with packaging films most of which are heat-shrinkable, the white line mark remains distinct after heat shrinkage, which is unfavorable to use as packaging films.